VALVE BEING PROBED IN ROUGH JET LANDING

The possible failure of a hydraulic system valve emerged Wednesday as a suspect in the investigation of a harrowing landing of an American Airlines passenger jet at O`Hare International Airport.

The incident, which occurred Tuesday evening, bore similarities to the troubled landing of an American passenger jet in May. That plane`s crew lost partial control of the aircraft after a failure of a similar valve, according to federal records. American has had at least four malfunctions of the same valve in other aircraft, the records reveal.

American Flight 1271, an MD-80 twinjet carrying 107 passengers on a trip from Rochester, N.Y., to Chicago, touched down at O`Hare at 5:45 p.m. and traveled about 4,500 feet before it swerved off the runway.

The plane traveled another 1,000 feet in the grass when the nosewheel struck a manhole cover and apparently collapsed, said Alan Pollock, a spokesman for the National Transportation Safety Board in Washington. The plane then slid an additional 550 feet and came to rest in mud, he said.

Pollock said that the valve on the O`Hare plane ''is one of the things

(investigators) are looking at very closely'' as they seek to pinpoint the cause of the mishap.

''The potential loss of hydraulic power in MD-80 series airplanes poses a significant safety problem,'' the head of the safety board said in a letter sent to the Federal Aviation Administration Aug. 31, after an investigation into the May incident.

''Of particular concern . . . is the effect that hydraulic system failure would have on landing performance, as a result of the loss of the use of flaps and normal wheel brakes,'' James Kolstad, acting chairman of the safety board, asserted in the letter.

The suspect component, called the ''power transfer unit shutoff valve,''

is ''a backup unit in the event of a hydraulic failure,'' said Keith Takahashi, a spokesman for McDonnell Douglas Corp., the MD-80`s manufacturer. ''However, an MD-80 can fly even with the loss of hydraulics,'' he said.

''You still have control through the mechanical system. . . . So we are kind of at a loss right now as to what happenened in Chicago. We just have to wait to see what further facts the (safety board) uncovers.''

The manufacturer of the valve, Whittaker Controls, has redesigned the component because of failures, officials said. McDonnell Douglas has recommended that MD-80 owners install the new valve and has itself begun putting it in planes on the production line, according to safety board officials.

American Airlines has begun ''a program of putting modified parts into that control valve system'' on its fleet of 165 MD-80 series aircraft, but ''I understand that the plane (at O`Hare) did not have a modified part in yet,''

said Tom Stack, an airline spokesman.

But, Stack added, ''We don`t have any final determination at this point on what, if any, relationship that had to the incident.''

The safety board has called for the FAA to make installation of the improved valve mandatory.

FAA officials have been considering the recommendation ''and expect to take corrective action in a week,'' said Fred Farrar, an agency spokesman in Washington.

The O`Hare jet was put into service on Sept. 21, 1984, Stack said. It had undergone several inspections in the last three weeks, including a so-called

''B-check'' on Aug. 26, he said.

B-checks are conducted about every 45 days and involve roughly 100 hours of work, Stack said. The hydraulic valve ''is the kind of item that would be handled'' during such an inspection, he said.